Congrats to Sen. Chuck Schumer, who’s now on the cusp of becoming Senate Democratic leader, with an excellent chance of being majority leader.

Too bad that success only raises the stakes in his coming crisis of conscience over President Obama’s Iran deal.

Sen. Harry Reid’s announcement Friday that he’ll retire come the end of 2016, and his endorsement of Schumer to be their party’s next Senate leader, leaves Chuck — a lifelong legislator — poised to achieve the pinnacle of legislative power.

Not just minority leader: Democrats are in fine shape to win the Senate majority in 2016, ’18 or ’20.

But Schumer could wind up a leader badly out of step with his caucus if he does the right thing on Iran.

Just this week, Schumer signed on as a co-sponsor of the bipartisan Corker-Menendez bill, which would force President Obama to submit the text of his Iran deal to the Senate for approval within five days of it being inked — and prevent him from easing sanctions on Iran for 60 days.

The bill will pass; Obama will veto it. Will Schumer vote to override — and help mark the start of the lame-duck era of this presidency?

The deal is going to be awful: At best, Iran will promise not to actually go nuclear for a few years, in exchange for the rapid repeal of US and UN sanctions and a de facto legalizing of all the work it’s done building nukes in violation of its past promises.

Let’s be clear here: This is not remotely all about Israel.

If Iran goes nuclear — or is plainly left free to go nuclear — the Saudis will follow suit. Egypt and others won’t be far behind.

The Saudis are at war now on their southern border in Yemen, bombing Iran’s Houthi pawns. Egypt’s getting in the mix, too, and there’s no end in sight.

Add nukes to the equation, and a huge chunk of the world’s oil supply could soon be glowing in the dark.

Unlike some Democrats, Schumer is no geopolitical naïf. He’s well aware of all this, and of the need to put an end to Obama’s dunderheaded diplomacy. He’d just rather someone else got the blame.